Recently, electronic devices (e.g., MP3 players, digital cameras, mobile phones, digital camcorders, PDAs, notebooks, and the like) are being made smaller in size for being more easily carried. With such trend, studies on a secondary battery as a source of power supply capable of being smaller and lighter for being more easily carried and more durable, in particular, a Li-secondary battery, have been steadily progressing. For such source of power supply, if a battery as a thin layer having a thickness of a few tens of nm to a few μm can be integrated inside of a micro device, it can be widely used in various fields such as MEMS (Micro Electro Mechanical System), micro robotics, micro sensors, and the like as well as in portable devices.
Meanwhile, considering the depletion of petroleum resources, environmental pollution, etc., many efforts have been continuously made towards replacing existing internal combustion engine automobiles with electric vehicles (EV) or hybrid electric vehicles (HEV). For this, there is a need for a battery having a high power density, excellent stability and low cost.
A battery includes a cathode, an anode, an electrolyte and a separator. Among these, the active material of each of the cathode and the anode is the most influential factor on battery characteristics.
In general, for a cathode material of a lithium secondary battery, J. B. Goodenough et al. (US) introduced a design using a LiCoO2 positive active material having a layered structure in 1980. Since then, the cathode material of the lithium secondary battery was first commercialized by SONY (Japan) in 1991 and is widely used till now.
There are various materials for the anode, such as lithium metal, a lithium metal alloy, a carbon material, silicon, tin oxide, a transition metal oxide, and the like. However, a carbon material which is low in potential change with respect to an intercalation and emission reaction of lithium and has an excellent reversibility, has become commercialized.
However, the carbon material (graphite) currently being commercialized and used intercalates one lithium atom per 6 carbon atoms (LiC6) in theory, limiting the theoretical maximum capacity to 372 mAh/g and thus limiting an increase in its capacity.
In addition to the carbon material, when lithium is used, a high capacity can be implemented due to a high energy density. However, dendrite formation due to the strong reducing power of lithium causes problems related to stability. Due to the intercalation and deintercalation processes of lithium, cycling characteristics are greatly reduced.
Silicon, tin or alloys thereof are being studied as alternatives. Silicon undergoes a reversible reaction with lithium through a compound formation reaction with lithium and has a theoretical maximum capacity of 4200 mAh/g, which is a greatly higher value compared to that of a carbon-based material. Further, tin oxide also has a high theoretical capacity (bulk SnO2, 1494 mAh/g). However, a very great volume change of 200-350% occurs due to a reaction with lithium when charging/discharging, thereby causing the separation of the negative active material from the collector during repetitive charging/discharging cycles or deteriorating the cycling characteristics due to an increase in resistance according to the change in contact interface among the negative active materials.
To overcome these problems, Korean Laid-Open Patent Publication No. KR10-2007-0005149 disclosed a tin-based nano-powder capped with a monomer which is used as a negative active material for a secondary battery of high efficiency, showing excellent cycling characteristics. However, expensive nano-powder of 10 nm-300 nm should be used and organic coating is needed, thereby entailing high fabrication cost and making the process complicated.
Further, Korean Laid-Open Patent Publication No. KR10-2005-0087609 disclosed that when a small amount of SnO2 is added into carbon powder used as an anode material of a conventional lithium secondary battery, high reversibility and excellent cycling characteristics were obtained. However, it was observed that the cycling characteristics were progressively reduced after 10 charging/discharging cycles.
Further, in order to minimize such volume changes and obtain high capacity values, studies on growing a SnO2 nanowire by a thermal evaporation of metal Sn have been made by Z. Ying et al. [Characterization of SnO2 nanowires as an anode material for Li ion-batteries, Applied Physics Letters, 87, 113108, 2005]. However, a significant amount of capacity decay from 1250 mAh/g to 700 mAh/g was observed from the 2nd cycle to the 15th cycle.
Recently, interest is highly increased in electrospinning as a fabrication method of a functional nano-fiber. Nano-fibers fabricated by electrospinning have a high porosity and an enhanced surface to volume ratio thus to be expected to have enhanced physical properties. However, in the case of a metal oxide/polymer composite fiber obtained by electrospinning, while polymers are decomposed, in a thermal treatment process at a high temperature of more than 450° C., it is observed that a rapid volume contraction is accompanied and cracking or separation from a lower substrate on which the composite fiber is formed occurs.
Accordingly, when a metal oxide such as SnO2 is used as a negative active material, there is a need for a composition with a metal oxide having a new structure which can minimize a volume change when charging/discharging and highly enhance the cycling characteristics. Also, there is required a composition with a metal oxide which can be applied to a thin film or a thick film with comparatively low cost processes and rapid yield. Further, by increasing adhesion with the substrate, enhancement of stability and high output density can be obtained. Accordingly, a next generation negative active material having a high possibility of commercialization needs to be developed by highly improving the “rate performance” which is considered to be a weakness of common negative active materials.